Many efforts have been made to develop successful logging while drilling systems, as suggested by the following examples: Karcher, U.S. Pat. No. 2,096,279 proposes a system utilizing electrical conductors inside the drill pipe. Heilhecker, U.S. Pat. No. 3,825,078 proposes a system utilizing extendable loops of wire inside the drill pipe. Silverman, U.S. Pat. No. 2,354,887 proposes a system utilizing inductive coupling of a coil or coils with the drill pipe near the drill bit with measurement of the induced electrical potential at the earth's surface. Arps, U.S. Pat. No. 2,787,759 and Claycomb, U.S. Pat. No. 3,488,629 propose systems in which pulsed restrictions to the drilling mud flow produce pressure pulse signals at the earth's surface. Other related U.S. Pat. Nos. are 3,186,222, 3,315,224, 3,408,561, 3,732,728, 3,737,845, 3,949,354 and 4,001,774. All of the foregoing patents are specifically incorporated into this specification by reference.
Each of the abovementioned proposals has had some drawback of sufficient consequence to prevent its commercial acceptance. For example, the inconvenience and time involved for the large number of connections and disconnections of electrical connectors is a significant drawback in systems such as proposed by Karcher. Though an induced electric potential system such as proposed by Silverman may be considered operable for a short distance, the signal to noise ratio of such a system prohibits its use as a practical matter in deep wells.
When modern jet bit drilling became commonplace and very large mud volumes and high mud pressures were employed, the systems as proposed by Arps, proved to be unreliable and subject to rapid deterioration. The introduction of a controlled restriction into the very powerful mud stream, of necessity, required large and powerful apparatus and operation was unsatisfactory because of rapid wear and very high energy requirements.
The environment is very hostile at the bottom of a well during drilling. Drill bit and drill collar vibrations may be in the order of 50 g. The temperature is frequently as much as 400.degree. F. The bottom hole pressure can be more than 15,000 psi. The drilling fluid flowing through the drill collars and drill bit is highly abrasive. With present drilling equipment including improved drill bits, the continued drilling time with a particular bit can be in the order of 100-300 hours and sometimes longer before it becomes necessary to change the drill bit. Accordingly, a downhole formation condition sensing and signal transmitting unit mounted near the drill bit must be capable of operating unattended for long periods of time without adjustment and with a continuing source of electrical power. Also, the signal communication apparatus must be capable of transmitting a continuing usable signal or signals to the earth's surface after each additional joint of drill pipe is conventionally added to the drilling string as the drilled borehole is increased in depth.
In general, systems using mud pulsations for telemetry are considered the most practical since the drilling operation is least disturbed. To date, however, the reliability that has been achieved with such systems is not satisfactory. The previous methods such as those of Arps and Claycomb, utilize the insertion of a controlled restriction into the mud flow circuit. However, when the mud flow surpasses 600 gpm and pump pressures pass 3000 psi, controlling this large energy by varying a restriction to produce telemetry signals is complicated and requires powerful downhole machinery.
A general objective of the present invention is to provide a successful logging while drilling system of the type utilizing mud pulsations for telemetry to transmit signals representing one or more downhole parameters to the earth's surface.
More specifically, it is an objective of the invention to provide such a system wherein the amount of energy that is required to generate a strong pressure pulse at a tool near the drill bit is significantly reduced.
Another objective of the invention is the utilization of an existing large source of energy for the production of the mud pulsations.